1. Field of the Invention
The present invention relates to the art of calibrating beam emitter type speed sensors, and more particularly relates to a method for calibrating such a speed sensor at line-of-road speeds for railroad rolling stock without the necessity of adjusting the antennae, their housing or their signal processing circuit.
2. Description of the Prior Art
The first overland vehicles to attain remarkable speeds of more than 30 miles per hour over sustained distances were railroad rolling stock in the early and middle Nineteenth Century. Usually, passenger and freight rolling stock powered by steam locomotives were carried over parallel rails mounted on a track bed for long distances. For the convenience of passengers and shippers, the times between stations were often the measure of speed. In the late Nineteenth and early Twentieth Centuries, the great railroad companies in Great Britain staged celebrated races of their trains from London to various points in Scotland often 500 miles away. In America, speed records between various American cities were continually being re-set.
For all that, it was unusual to find a speedometer or speed sensor in a locomotive during those times. Tuplin, in The Steam Locomotive (1974), Charles Scribner's Sons, New York, describes that during these times the British drivers were not officially required to carry even a watch, but got along by adjusting their speeds on the basis of their long experience, with some guidance from station clocks and other clocks as they might see from the cab during a trip. The guard of the train was required to carry a watch since he had to record passing times at certain places and to notify the driver at the end of a journey with a signed "ticket" noting time gained or lost according to the pre-announced schedules.
Given the present conditions of certain railbeds along various rail routes in the United States, many carriers do not operate a railroad train at open speed in the present era, and ambitious schedules are frequently abandoned for the sake of safety. Indeed, many states or municipalities have enacted "slow orders" in the form of railroad regulations in order to regulate the speeds of trains over certain sections of track within their jurisdictions. Such regulations are in response not only to roadbed conditions, but also to population density and railroad traffic conditions that might well affect the safety of railroad operations.
In addition to governmental regulations, labor organizations have begun to insist on accurate and operable speed indicators in locomotive cabs. These demands have led one expert in the field to foresee an engineman's right to refuse a consist when the speed indicator is inoperable or inaccurate. See Stringer, Chairman, in the "Report of the Committee on Diesel Electrical Maintenance" before the Chicago Railroad Diesel Club (Sept. 17, 1979).
It is common now for permanent records to be made from the speedometer readings so that the decisions and actions of the locomotive operator can be reconstructed. Such information is invaluable in planning route schedules, routing and in reconstructing problems immediately prior to traumatic disruptions, such as derailments, grade crossing accidents and the like.
In the past, railroad speedometers have taken their reading off of the axle itself. Such a system would be accurate only if there was little slippage of the wheel on the rails; thus significant attention was given to preventing such slippage and loss of adhesion. Certain new locomotive designs, however, will employ new technological concepts that tolerate significant slippage of the wheel on the track. Since the indicator and the speed recording systems are designed to monitor the actual ground speed, such axle derived speedometers will not be sufficient.
Stringer, supra, in his address in late 1979, indicated that radar may successfully eliminate the axle driven speedometer and is something new in the area of electronic speed indicators.
In order to determine the speed in such radar speed sensors as indicated in the prior art, it is necessary to maintain a very precise angle or alignment of the radar beam to the ground underneath, or to provide two ground directed radar beams precisely aligned with respect to each other in order to reference the speed indicating signals in one radar beam against a reference signal of the other radar beam. Thus, the reading of the speed sensing device is dependent upon maintenance of one radar gun or antenna at a precise angle either to the carriage of the locomotive, or to a second reference radar or antenna gun. When the radar gun or antenna beam path and its angle to a reference are changed, the accuracy of the sensed speed deteriorates.
Several Doppler antenna speed indicating devices are known in the prior art for aircraft, as seen representatively in U.S. Pat. No. 2,426,228 to L. Mackta; for ships, as seen representatively in U.S. Pat. No. 3,277,430 to J. Hagemann and D. H. Brunk; and automobiles, as seen representatively in U.S. Pat. No. 4,107,680 to Kaplan and U.S. Pat. No. 3,859,660 to C. F. Augustine and R. E. Anderson. All of these applications are substantially different from those of railroad rolling stock in that railroad rolling stock is subjected to environmental conditions of extreme vibration and shock, substantially more than that in aircraft, ships and automobiles. See Stringer, "Report of the Committee on Diesel-Electrical Maintenance" (Sept. 17, 1979), supra at 155. For examle, a two degree change in this angle of the Doppler antennae, originally set at 45 degrees with respect to the roadbed, results in a 3.5% error in the accuracy of the detected speed. Consequently, it has been found that one cannot simply adapt the Doppler antenna or radar speed sensing devices of the prior art to railroad rolling stock because the antennae are subject to shock and vibration-induced misalignments which intermittently change the readings of the sensors and hence, directly cause inaccuracy in the speed readout from the speed sensing device.
Sometimes the speed and distance traveled of the locomotive is recorded on graphs, and used with other data for important calculations. Over the long distances of train routes, small speed accuracy deterioration can result in significant cumulative distance errors. The severe shock and vibration of the locomotive, moreover, are not the only factors which will change the mounting of the antennae. The air turbulence under the locomotive over the railroad gravel bed stirs gravel and debris, and animals and the like frequently knocking about. Such debris can knock the radar antennae out of mounting alignment.
Periodic calibration of the speed sensor is required, therefore, for reliable speed readouts from such a beam emitter type speed sensor. Alignment requires substantial time and maneuvering of someone under the locomotive. Such alignment necessarily requires manual leveling which very likely cannot be sufficiently rigorous to establish the accuracy desired.
A Doppler antenna calibration apparatus is shown in U.S. Pat. No. 3,787,866 to G. R. Gamertsfelder, Stavis and Vladimir. Such a calibration apparatus requires that the antennae apparatus be detached from its aircraft vehicle and mounted in a special alignment mounting relative to an endless belt, and in very close proximity to the endless belt to provide an accurate reading. After calibration, the speed sensor must be remounted in the vehicle with matching good alignment.
In the case of railroad rolling stock, the antenna would have to be remounted on the railroad vehicle with very good initial alignment accuracy and would have to maintain such alignment throughout long runs of the vehicle. It is desired, therefore, to provide a method which does not depend upon precise alignment but instead provides for readily recalibrating such radar speedometer antennae mounted on the undercarriage of railroad rolling stock. It is also desired to provide a method of recalibrating such radar speedometer antennae without adjusting the antenna mounts or without moving or touching the antennae at all, and to perform such calibration by a railroad electrician acting alone within the locomotive cab.